A conventional submarine repeater structure for ocean observation has a structure which comprises a pressure gauge which determines an external pressure by detecting a frequency difference between a pressure-sensitive crystal and a reference crystal. In general, the pressure-sensitive crystal is disposed on the side of sea water, and the reference crystal is disposed in a pressure tight box to be separated from the pressure-sensitive crystal.
However, in the conventional submarine repeater structure, there is a problem that a precise measurement of pressure may be not always obtained since the two crystals are disposed in the respective positions with different temperature conditions. In particular, the frequency of the reference crystal in the pressure-tight box tends to be varied due to the thermal affection by the thermogenesis of electronic circuit.